Light Emitting Diodes (LEDs) take many different forms and are increasingly used in varied applications ranging from display backlighting, to traffic lights, to street signs, to indicator lamps, to general lighting, to UV curing of adhesives, to spectroscopy, and to water purification. In recent years, there have been significant technical advances leading to improved LED brightness. However, the ability to efficiently extract light out of LED structures has typically been a challenge in the design of high efficiency LEDs.
In the case of nitride-based UV LEDs, light is usually extracted out of the backside of the wafer because one or more layers above the light generating active layer are typically absorbing. Moreover, layers having high aluminum content in the nitride system are exceedingly resistive, so a current spreading top layer positioned above the current injection area of the wafer is typically used. Nitride-based light-emitting diodes (LEDs) usually feature such a top layer that fully covers the current injection area. This top layer can be an opaque metal or a transparent Indium Tin Oxide film. Its purpose is to spread current across the entire current injection area. It is also desirable to make these top layers highly reflecting, so upward going light can be reflected downward towards the backside or bottom output.
However, highly reflective top layers are difficult to attain. A good p-type contact requires that the noted current spreading metal contact layer that covers the entire current injection area of the device be alloyed to the p-type layer. The rough interface between the alloyed contact and the p-type layer causes large amounts of light scattering and absorption.
FIG. 1 shows a top view of a typical nitride-based LED 10 having a top p-contact layer 12 having a metal contact 14 formed thereon and an n-contact layer 16 having an n-contact pad 18. The p-contact layer 12 is covered by the metal contact 14 which is typically formed of a Ni/Au metal alloy formed thereon. This metal contact 14 is formed by initially depositing Ni and Au as separate metal layers. The layers are then annealed to alloy them into an underlying GaN epitaxial contact layer (not shown in FIG. 1). The metal contact 14 serves as a current spreading layer that distributes current across the entire current injection area. The alloyed interface between the metal contact and the underlying epitaxial layer allows electrical injection into the device. Unfortunately, as noted above, the alloying process results in a rough interface that scatters light and leads to low top-side reflectivity.
FIG. 2 shows a cross section illustration of the device 10 in FIG. 1. Light generated at an active region (e.g., the InAlGaN heterostructure multiple quantum well layers) within the epitaxial layers 20 is reflected by the top metal contact 14, so light has to be emitted out of the structure through the wafer backside (e.g., sapphire substrate 22 and AlN template 24). In the case of ultraviolet (UV) LEDs, a portion of the upward-directed light would be lost through absorption because the top contact layer 12 is absorbing. Although higher bandgap materials would be transparent to UV light, they are not well suited as contact layers because high aluminum-containing p-type AlGaN films are very resistive.
In connection with other technologies such as GaAs surface-emitting laser devices, the use of annular rings is not uncommon. However, for nitride based light emitting diodes contemplated above, if an annular ring were used as a top metal contact, the highly resistive upper p-type layers would prevent current from spreading into the center region of the device under the “hole” of the ring. Most of the current would be channeled into the area directly below the annular ring portion where the metal is alloyed to the layer below. This poor current distribution would result in most of the light being emitted from only those areas. Of course, it is desirable to emit light from the entire p-contact area, not just from areas below the location of the metal.